1. Field of the Invention
This invention relates to a wavelength conversion device employing a non-linear optical crystal, such as .beta.-barium borate crystal. More particularly, it relates to a wavelength conversion device used advantageously in, for example, a semiconductor light exposure device that requires uniform light intensity distribution.
2. Description of the Related Art
In a wavelength conversion device for wavelength conversion by generating second harmonics, the fundamental wave is converged in a non-linear optical crystal in order to effect wavelength conversion by a non-linear optical process in the non-linear optical crystal.
Heretofore, if, in a wavelength conversion device, the input fundamental wave has a Gaussian intensity distribution, and the non-linear optical crystal has a pre-set walk-off, the optimum convergence condition for wavelength conversion has been found by an equation by Boyd and Kleinman, `Journal of Applied Physics`, band 10, page 3597, 1968.
However, the Boyd and Kleinman's equation neglects attenuation of the fundamental wave in the non-linear process, that is depletion. Thus, if the depletion of the input fundamental wave is not negligible, specifically, if the power of the input fundamental wave is high, approximation is deteriorated significantly.
Meanwhile, if the fundamental wave of a power for which depletion is not negligible is input to a non-linear optical crystal, the intensity distribution of the waveform-converted light is distorted significantly, by the synergistic effect of depletion and walk-off, thus occasionally leading to significant peaks in the light intensity in the non-linear optical crystal.
Such peaks in the light intensity directly give rise to non-linear absorption and to light damage to the non-linear optical crystal. Moreover, if there is significant distortion in light intensity distribution in the non-linear optical crystal, the conversion efficiency is undesirably lowered when the second harmonics is used as a light source for the wavelength conversion of the next stage. Thus, in a wavelength conversion device, it has been desired to eliminate distortion in light intensity distribution in the non-linear optical crystal and any significant peaks in the light intensity.
However, since the Boyd and Kleinman's equation used for designing the wavelength conversion device does not take account of depletion, it has previously not been possible to analyze the phenomenon derived from the synergistic effect of the depletion and the walk-off.